1. Technical Field
The present invention relates to microchip fabrication techniques which enable precise periodic assembly of semiconductor materials. The advanced thin film products are thermoelectric or micro-electromechanical (MEMS) devices.
2. Description of the Related Art
Although the thermoelectric effect for cooling, heating, or power generation was discovered over a century ago, currently available thermoelectric devices that capitalize on the effect are limited. Suitable materials for fabricating useful devices are difficult to make or to buy. Existing assembly processes have not produced devices of a practical size. The thermoelectric devices use semiconductor materials to transport heat through steady state electron transport from junction to junction. The thermoelectric effect states that electrical current carries charge as well as heat flux. Cooling occurs in a device when electrons pass from a P-type semiconductor doped to have a deficiency of electrons to an N-type semiconductor (absorbing heat) doped to have an excess of electrons and then pass from the N-type semiconductor to the P-type semiconductor (dissipating heat). A temperature differential or gradient, known as the Peltier effect, arises across the device. If a natural temperature gradient occurs across the device, the device can produce an electrical potential suitable for producing electrical power according to the Seebeck effect. The power generating capacity in such a case is a function of the temperature difference across the device.
The thermoelectric effect has been implemented in commercial devices for only a limited number of applications where the energy efficiency or the maximum achievable temperature difference are not overriding concerns. The thermoelectric devices have found niche markets where the cooling power requirement is small (e.g. milliwatts to watts), such as portable food coolers, laser diode coolers, or infrared detector coolers. Traditional fabrication methods cut bulk thermoelectric materials into small devices, and then connect the devices on thermally conductive face plates. The devices have small areas normally less than 25 cm.sup.2 and processing methods limit the minimum device footprint to about 0.5 cm.sup.2. Low efficiency arises because the materials must compromise between the optimal electrical resistance and optimal thermal conductance.
As a practical matter, the Peltier and Seebeck effects have been unable to convert electrical power into practical, usable cooling capacity or, conversely, to convert thermal power (i.e., a natural temperature gradient) into electrical power because of the electrical resistance and thermal conductance of the semiconductor material. These factors limit the device efficiency. Additionally, presently available commercial manufacturing processes limit the device size to dimensions much larger than are compatible with the sizes of devices where such heating or cooling could be beneficially applied. Advanced thin film thermoelectrical materials such as quantum well, Skutterudites, and doped C.sub.60 (Fullerine) materials would allow exploitation of the thermoelectric effect at efficiencies comparable with Carnot cycle engines. Such thin film materials use multilayer quantum confinement, and superlattice structures. They are thin and physically weak compared with standard thermoelectric semiconductor materials. The advanced thin film materials are not strong enough, however, to act alone as structural elements within thermopiles. If low cost, reproducible, high precision methods were available to make devices from these advanced films, a broad range of applications would arise capitalizing on the capabilities these films provide, such as flexibility, variable packing density, and multiple staging.
Micro-electro-mechanical System (MEMS) technology allows the manufacture of mechanical parts having sizes on the order of microns. Microtransducers can out perform traditional (macro) transducers in many applications by orders of magnitude because of their smaller size. The potential for applying MEMS to aerospace engineering is discussed, for example, in the article: Ho, et al, "MEMS--A Technology for Advancements in Aerospace Engineering," 35th Aero. Sci. Mtg., AIAA-97-0545, Reno, Nev. (Jan. 6, 1997), which we incorporate by reference. Micro-sensors and Micro-actuators have potential for controlling shear stress or for providing flight controls.